The present invention relates to golf clubs that offer improved performance due to their novel design.
All golf clubs with a configuration comprising grip, shaft, and head with a face, heel, toe, sole, and crown—meeting USGA regulations—will have locations on the face between heel and toe, extending from sole to crown, which can be designated as centers of percussion. A line which may be termed a “line of centers of percussion” can then be drawn on the clubface connecting these points from sole to crown. It will be found that this line is parallel to the shaft axis (i.e., centers of percussion are at constant radius from the shaft axis at all elevations on the clubface).
This terminology of “center of percussion” is very specific as described in Wiley's, “Engineer's Desk Ref.”, 1984, page 96 (which page is herein incorporated by reference), and should not be confused with center of gravity, geometric center, or any other “center” description that might be loosely used or in fashion. Its significance lies in the fact that it is the ONLY location between heel and toe—at a particular elevation—which defines the point at which ball impact will not produce shaft twisting. Impacts between this point and the heel of the club will produce counter-clockwise shaft twisting (from the golfer's viewpoint), and impacts between this point and the toe of the club will produce clockwise shaft twisting. Both are undesirable.
The reason for this performance is as follows. For any given impact point the clubhead can be divided into two sections: an inboard section and an outboard section. The inboard section is defined as the section running from the heel to a line on the clubface parallel to the clubshaft and running through the impact point. The outboard section is defined as the section running from the toe to the same line. The line of centers of percussion is established by the outboard and inboard sections having identical moments of inertia about the clubshaft axis. These results come from a dynamic action in which the ball (or ground) impact produces an abrupt clubhead deceleration focused at the point of impact. This in turn causes the mass of each of the inboard and outboard sections to want to continue its motion at its original speed and direction because of its inertia. Whereas the static concept of center of gravity is defined by weight at a linear distance from the center of gravity, the dynamic center of percussion is defined by mass times the shortest distance from the axis of concern squared (I=md2, where I is the moment of inertia). An Impact point on the line of centers of percussion will yield a moment of inertia about the shaft axis for the inboard section equal to that for the outboard section and will result in no shaft twisting. Conversely, impact points at locations other than a center of percussion will then produce imbalances between the moments of inertia of the inboard and outboard sections and will result in shaft twisting.
A critical feature of the center of percussion which distinguishes it from the static center of gravity, for example, is that it remains immutable from earth to space. That is, impacts on a clubface under testing will yield exactly the same shaft twisting for the same impact location whether on earth or in space (zero gravity environment). Since there is no such thing as “center of gravity” in outer space at zero gravity, this demonstrates that the singular parameter governing shaft twisting is the center of percussion, NOT the center of gravity or other so-called “center”.
It is recognized that in an earth environment, changing the center of percussion by changing mass distribution will also change the center of gravity. However, this does NOT make them one and the same parameter. One is a static parameter, and the other is a dynamic parameter immutable from earth to outer space and singularly relevant to golf equipment and playing practice. Further, with an irregular three-dimensional body it is impossible to demonstrate proof of the location of the static center of gravity, but proof of a center of percussion location can be demonstrated practically.
The ball is not the only body that impacts the clubhead during a game of golf, and shaft twisting can also be caused by ground contact of the clubhead. Again, this twisting is undesirable and should be minimized or eliminated for improved performance of a new golf club design.
It would also be very useful to provide golfers with a new golf club specifically designed to have a line of centers of percussion located near the centroid of the clubface. Additionally, it would be very useful to clearly mark on the clubface the location of the line of centers of percussion so that the golfer can attempt to strike the ball on this line at all ball elevations, thereby preventing shaft twisting. Further, it would also be very useful to provide a new golf club design that minimizes shaft twisting due to ground contact of the clubhead.
The main objective of this invention is to minimize golf club shaft twisting when impacting both ball and ground. This objective can be realized for all club/shaft configurations which meet USGA regulations, and it is achieved by providing two design features which are uniquely coupled together in a preferred embodiment:    1. A clubhead design having a line of centers of percussion—manipulated by design, testing and manufacture—which goes through the centroid of the clubface area, and having a “permanent” stain on the clubface showing the location of this line of centers of percussion for the golfer's guidance—at ALL ball elevations from the ground. This design is achieved by manufacturing the clubhead such that a line going through the centroid of the clubface and parallel to the club shaft axis, divides the clubhead into two sections of substantially equal moments of inertia about the clubshaft axis. By definition this line is then the line of centers of percussion and its location can be marked on the clubface with a permanent stain. It is important to note that:    a. No golf club manufacturer has ever offered by design a center of percussion or line of centers of percussion—visible or otherwise—on their clubs.    b. No golf club manufacturer has ever located by design a center of percussion or line of centers of percussion near the centroid of face area so as to offer improved surrounding area tolerance for misshits. (Current clubs, irons in particular, have a line of centers of percussion—as discovered by random testing—very close to the hosel which can easily result in the dreaded shank from a modest misshit.)    2. A sole profile (front elevation view) in the form of an inverted peak roof, the objective of which is to provide one singular point—the peak—which will make initial contact between sole and ground.
The unique coupling of this feature with the previous item (1) is then achieved by locating this peak substantially coincident with a point on the line of centers of percussion. In short, the line of centers of percussion preferably goes through both the centroid of the clubface and the peak. In this way ground impacts will act like ball impacts on the line of centers of percussion and will produce no shaft twisting. This is a truly superior arrangement. An inverted peak roof type sole profile has been in common use for some clubs, particularly the “woods”. However, the foregoing unique coupling has never been attempted prior to this invention. Nor has anyone made an inverted peak roof profile with OTHER than flat surfaces from peak to toe and peak to heel—all potentially desirable configurations for the purposes of this invention.